Gently weighing viruses
More accurate measurements agree with early studies
How much does a virus weigh? According to a research team from Academia Sinica, from National Taiwan Normal University and from the National Defense Medical Center, all in Taipei, Taiwan, as well as from Wuhan University in China — not much. The exact value depends upon the kind of virus. The human adenovirus type 5, for example, tips the scales at 172 MDa, or about 286 billionths of a billionth of a gram. That is about a thousand times smaller than the mass of a bacterium.
These results are in agreement with earlier ones, but earlier measurements had larger uncertainties. The new ones have an error of only a few percent, in part because the researchers used different techniques. “This is the first time that such a measurement was conducted using photonic detection methods,” said team leader and Academia Sinica research fellow Huan-Cheng Chang.
Researchers weighed a virus using a laser (blue box, lower right) that turns virus particles into an aerosol through laser-induced acoustic desorption (LIAD). The free-floating virus particles are then transferred into a cylindrical ion trap (CIT), where a second laser illuminates them. The motion of individual particles is tracked with a CCD and their frequency spectrum measured with a photomultiplier tube (PMT) (a). The result, in (b), (c) and (d), is ultimately the mass of the particle, expressed here in megadaltons for polystyrene beads of 100-nm nominal diameter. Images courtesy of Huan-Cheng Chang, Academia Sinica.
He added that two photonics-related innovations played key roles. The first was laser-induced acoustic desorption, which gently converted the viruses into an aerosol by inducing sound waves in a plate bearing the virus sample. The investigators did this by firing a frequency-doubled 532-nm Continuum Nd:YAG laser into the back of the sample plate. The viruses were popped free by sound waves and, as a result, rendered gaseous intact, despite being made of relatively soft material. The work was detailed in the Dec. 11, issue of Angewandte Chemie International.
Once freed, the viruses were charged and caught in an ion trap. In their work, the researchers used a cylindrical ion trap constructed of a stainless-steel barrel and two flat indium tin oxide-coated glass plates for end caps. The plates were transparent and conductive, enabling the second photonics-related innovation. Because the end caps were flat and transparent, the researchers captured more than 10 percent of the light scattered by particles in the trap.
The ion trap approach is not new, but the shape is. Previously, the researchers used a standard quadrupole ion trap, determining the mass of a single cell with an accuracy of better than 1 percent. Unfortunately, the hyperbolic shape of the electrodes made light collection difficult and limited the minimum size of the particles that could be detected to 300 nm. Use of the cylindrical ion trap and associated innovations enabled the investigators to increase the detection sensitivity a thousandfold, allowing them to measure much smaller particles.
One reason the cylindrical ion trap had not been used before is the motion of the particles, which must be tracked for measurements to be made. In the cylindrical trap, this movement is complex and cannot be solved analytically. However, the scientists did not have to account for all the possible particle motions. “We replaced the hyperbolic quadrupole ion trap by a cylindrical ion trap with the understanding that the particle should behave the same in the centers of both,” Chang said.
During operation of the instrument, the researchers employed a 532-nm Nd:YAG from Photop Technologies Inc. of Fujian, China, as a light source. They sent the beam into the trap and detected the scattered light. They used part of the scattered light for frequency spectrum analysis based upon measurements done with a Hamamatsu photomultiplier tube. With the rest, they performed particle trajectory imaging using an electron-multiplying CCD from Cooke Corp. of Auburn Hills, Mich.
In their instrument, the researchers used a high-performance CCD because, otherwise, they would not have been able to see the weak scattered light from a single trapped virus particle 80 nm or so in size. By capturing how the particle moved, they could determine its mass and, if the density were known, its size.
The setup used a long-working-distance objective from Mitutoyo America Corp. of Aurora, Ill. The 20-mm working distance and 0.42 NA of the objective made integration of the trap with an optical microscope possible, Chang noted.
Measuring the masses of viruses with diameters of less than 50 nm will require the use of interferometric methods. A key step is to combine the transparent cylindrical ion trap with an optical microscope, as seen in these photos.
The researchers proved their instrument using standard polystyrene beads of 100-nm nominal size from the Gaithersburg, Md.-based National Institute of Standards and Technology (NIST). Their measurements on the beads indicated an average mass of 332 MDa and a corresponding size of 100 nm. This result was in close agreement with the 99.7-nm average size certified by NIST.
They measured human adenovirus type 5, grouper iridovirus and vaccinia virus, whose sizes ranged from 80 to 300 nm and whose mass varied by two orders of magnitude. For the largest, the vaccinia virus, they obtained a mass of 3.26 GDa, which agreed with previous results. For the smallest, the human adenovirus type 5, they recorded a mass of 172 MDa and a size of about 80 nm with a variation of about 2 percent. This agreed with other results, but size distribution was five times as narrow as that of the other methods.
There are some drawbacks to the new approach, chief among them the 10-minute measurement time for a single virus. Other methods measure thousands of virus particles in approximately 30 minutes. The researchers are changing their setup to automate the process and shorten the time.
Another disadvantage is the lower limit to the size of particles that can be detected. Currently, that is about 50 nm, which makes it impossible to use the present instrument with the smallest viruses. The team plans to overcome that limit by employing interferometry-based detection methods.
Plots of measured masses versus particle numbers for grouper iridoviruses (a) and recombinant human adenoviruses (b) are shown. Upper insets are electron micrographs of negatively stained iridoviruses (about 170 nm in diameter) and adenoviruses(approximately 80 nm in diameter). Lower insets are mass distributions. A comparison of the two viruses suggests that 70 percent of the weight of the iridovirus is water.
Other improvements being put into place include the use of microscopic objectives with larger numerical apertures, which will improve performance but will require miniaturizing the ion trap because the working distance will shrink. Chang believes that the effort needed for such enhancements is warranted because of the possible payoff.
“To reach the goal of measuring the masses of smallest viruses with sizes in the range of 20 nm, the microscopic methods clearly offer the most promising approach,” he said.
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